1. Field of the Invention
This invention relates generally to an apparatus and method for delivery and vaporization of liquid reagents for transport to a deposition zone, e.g., a chemical vapor deposition (CVD) reactor. More specifically, the invention relates to a reagent supply system for chemical vapor deposition using dissimilar precursor compositions.
2. Description of the Related Art
In the formation of thin films, layers and coatings on substrates, a wide variety of source materials have been employed. These source materials include reagents and precursor materials of widely varying types, and in various physical states. To achieve highly uniform thickness layers of a conformal character on the substrate, vapor phase deposition has been used widely as a technique. In vapor phase deposition, the source material may be of initially solid form which is sublimed or melted and vaporized to provide a desirable vapor phase source reagent. Alternatively, the reagent may be of normally liquid state, which is vaporized, or the reagent may be in the vapor phase in the first instance.
In the manufacture of advanced thin film materials, a variety of reagents may be used. These reagents may be used in mixture with one another in a multicomponent fluid which is utilized to deposit a corresponding multicomponent or heterogeneous film material. Such advanced thin film materials are increasingly important in the manufacture of microelectronic devices and in the emerging field of nanotechnology. For such applications and their implementation in high volume commercial manufacturing processes, it is essential that the film morphology, composition, and stoichiometry be closely controllable. This in turn requires highly reliable and efficient means and methods for delivery of source reagents to the locus of film formation.
Examples of advanced thin film materials include refractory materials such as high temperature superconducting (HTSC) materials including YBa.sub.2 Cu.sub.3 O.sub.x ; wherein x is from about 6 to 7.3, the bismuth-strontium-calcium-copper oxide and thallium-barium-calcium-copper oxide superconductors of varying stoichiometry, abbreviated BiSrCaCuO and TlBaCaCuO. Barium titanate, BaTiO.sub.3, and barium strontium titanate, Ba.sub.x Sr.sub.1-x TiO.sub.3, have been identified as ferroelectric and photonic materials with unique and potentially very useful properties in thin film applications of such materials. Ba.sub.x Sr.sub.1-x Nb.sub.2 O.sub.6 is a photonic material whose index of refraction changes as a function of electric field and also as a function of the intensity of light upon it. Lead zirconate titanate, PbZr.sub.1-x Ti.sub.x O.sub.3, is a ferroelectric material whose properties are very interesting. The Group II metal fluorides, BaF.sub.2, CaF.sub.2, and SrF.sub.2, are useful for scintillation detecting and coating of optical fibers. Refractory oxides such as Ta.sub.2 O.sub.5 are coming into expanded use in the microelectronics industry; Ta.sub.2 O.sub.5 is envisioned as a thin-film capacitor material whose use may enable higher density memory devices to be fabricated.
Thin films comprising the Group II metal fluorides, BaF.sub.2, CaF.sub.2, and SrF.sub.2, are potentially very useful as buffer layers for interfacing between silicon substrates and HTSC or GaAs overlayers or between GaAs substrates and HTSC or silicon overlayers, and combinations of two or all of such metal fluorides may be employed in forming graded compositions in interlayers providing close lattice matching at the interfaces with the substrate and overlayer constituents of the composite. For example, a silicon substrate could be coated with an epitaxial layer of BaF.sub.2 /CaF.sub.2, SrF.sub.2 /CaF.sub.2, or SrF.sub.2 /CaF.sub.2 /BaF.sub.2, whose composition is tailored for a close lattice match to the silicon. If the ratio of the respective Group II metal species in the metal fluoride interlayers can be controlled precisely in the growth of the interlayer, the lattice constant could be graded to approach the lattice constant of GaAs. Thus, a gallium arsenide epitaxial layer could be grown over the metal fluoride interlayer, allowing the production of integrated GaAs devices on widely available, high quality silicon substrates. Another potential use of such type of metal fluoride interlayers would be as buffers between silicon substrates and polycrystalline HTSC films for applications such as non-equilibrium infrared detectors. Such an interlayer would permit the HTSC to be used in monolithic integrated circuits on silicon substrates.
BaTiO.sub.3 and Ba.sub.x Sr.sub.1-x Nb.sub.2 O.sub.6 in film or epitaxial layer form are useful in photonic applications such as optical switching, holographic memory storage, and sensors. In these applications, the BaTiO.sub.3 or Ba.sub.x Sr.sub.1-x Nb.sub.2 O.sub.6 film is the active element. The related ferroelectric material PbZr.sub.1-x Ti.sub.x O.sub.3 is potentially useful in infrared detectors and thin film capacitors well as filters and phase shifters.
In these and other applications involving the manufacture of microcircuitry and microelectronic devices, there is a growing use of new thin film materials and corresponding precursors to fabricate products. This trend is accompanied by the evolution towards increasingly smaller microelectronic feature sizes, in which chemical vapor deposition (CVD) is often the preferred deposition technique because of the conformality, composition control, deposition rates and microstructural homogeneity that are characteristic of such method.
Chemical vapor deposition (CVD) is a particularly attractive method for forming thin film materials of the aforementioned types, because it is readily scaled up to production runs and because the electronic industry has a wide experience and an established equipment base in the use of CVD technology which can be applied to new CVD processes. In general, the control of key variables such as stoichometry and film thickness, and the coating of a wide variety of substrate geometries is possible with CVD. Forming the thin films by CVD permits the integration of these materials into existing device production technologies CVD also permits the formation of layers of the refractory materials that are epitaxially related to substrates having close crystal structures.
CVD requires that the element source reagents, i.e., the precursor compounds and complexes containing the elements or components of interest must be sufficiently volatile to permit gas phase transport into the chemical vapor deposition reactor. The elemental component source reagent must decompose in the CVD reactor to deposit only the desired element at the desired growth temperatures. Premature gas phase reactions leading to particulate formation must not occur, nor should the source reagent decompose in the lines before reaching the reactor deposition chamber. When compounds are desired to be deposited, obtaining optimal properties requires close control of stoichiometry which can be achieved if the reagent can be delivered into the reactor in a controllable fashion. In this respect the reagents must not be so chemically stable that they are non-reactive in the deposition chamber.
Desirable CVD reagents therefore are fairly reactive and volatile. Unfortunately, for many of the refractive materials described above, volatile reagents do not exist. Many potentially highly useful refractory materials have in common that one or more of their components are elements, i.e., the Group II metals barium, calcium, or strontium, or the early transition metals zirconium or hafnium, for which no or few volatile compounds well-suited for CVD are known. In many cases, the source reagents are solids whose sublimation temperature may be very close to the decomposition temperature, in which case the reagent may begin to decompose in the lines before reaching the reactor, and it therefore is very difficult to control the stoichiometry of the deposited films from such decomposition-susceptible reagents.
When the film being deposited by CVD is a multicomponent substance rather than a pure element, such as barium titanate, lead zirconate titanate (PZT), lead lanthanum titanate (PLT), or the oxide superconductors, controlling the stoichiometry of the film is critical to obtaining the desired film properties. In the deposition of such materials, which may form films with a wide range of stoichiometries, the controlled delivery of known proportions of the source reagents into the CVD reactor chamber is essential.
In other cases, the CVD reagents are liquids, but their delivery into the CVD reactor in the vapor phase has proven difficult because of problems of premature decomposition or stoichiometry control. Examples include the deposition of tantalum oxide from the liquid source tantalum ethoxide and the deposition of titanium nitride from bis(dialkylamide)titanium reagents.
In recent years it has been widely recognized that for many of these emerging new thin film materials, there are no well-behaved gaseous precursors available (at ambient temperature and pressure). In such instances, the liquid delivery technique has become the predominant method of choice to controllably deliver liquid and solid precursors (typically metalorganic compounds) to a CVD process.
In the liquid delivery approach, the liquid or solid precursor is typically dissolved in a solvent, and the solution is stored, e.g., at ambient temperature and pressure. When the deposition process is to be run, the solution is transported to a high temperature vaporization zone within the CVD flow system, where the precursor is flash vaporized (along with the solvent), and the gas-phase precursor then is transported to the deposition zone, such as a chemical vapor deposition reactor, containing a substrate on which deposition of the desired component(s) from the vapor-phase precursor composition takes place.
The liquid delivery technique has been found to be extremely useful for deposition of multicomponent oxide thin films such as (Ba,Sr)TiO.sub.3, SrBi.sub.2 Ta.sub.2 O.sub.9 (SBT), (Pb,La) TiO.sub.3 (PLT) and Pb(Zr, Ti)O.sub.3 (PZT) for example. In CVD processes developed for these and other compounds, it is highly desirable to dissolve all the precursors in a single solution, and vaporize them simultaneously, following which the vaporized precursor composition containing the respective components is transported to the deposition chamber, as described above.
Liquid delivery systems of varying types are known in the art, and for example are disclosed in U.S. Pat. No. 5,204,314 issued Apr. 20, 1993 to Peter S. Kirlin et al. and U.S. Pat. 5,536,323 issued Jul. 16, 1996 to Peter S. Kirlin et al., which describe heated foraminous vaporization structures such as microporous disk elements. In use, liquid source reagent compositions are flowed onto the foraminous vaporization structure for flash vaporization. Vapor thereby is produced for transport to the deposition zone, e.g., a CVD reactor. The liquid delivery systems of these patents provide high efficiency generation of vapor from which films may be grown on substrates. Liquid delivery systems of such type are usefully employed for generation of multicomponent vapors from corresponding liquid reagent solutions containing one or more precursors as solutes, or alternatively from liquid reagent suspensions containing one or more precursors as suspended species.
The liquid delivery approach has found widespread application in the CVD formation of BST films, which may for example be carried out with the tetraglyme adduct of bis-tetramethylheptanedionato Ba (Ba(thd).sub.2 -tetraglyme, the tetraglyme adduct of bis-tetramethylheptanedionato Sr (Sr(thd.sub.2 -tetraglyme), and bis-isopropoxy bistetramethylheptanedionato Ti (Ti(OPr).sub.2 (thd).sub.2) being dissolved in a solvent that may contain butyl acetate, and with the resulting solution being pumped to a zone where it is vaporized at surfaces that are maintained at a temperature of approximately 230.degree. C. This technique has been found to result in BST films with excellent quality, which possess superior microstructural and functional properties.
The simplicity of such liquid delivery approach for CVD of BST has been fortuitous, because each component in this system of metalorganic precursors can be treated identically in the respective solution-forming, vaporization and transport steps of the process. Thus, in such a compatible system of multiple, well-behaved precursors, (i) the precursors can be dissolved in the same solvent with high solubility, (ii) the precursors maintain their identity in the single solution, without deleterious chemical reactions with the solvent or net ligand exchange with each other, (iii) the precursors can be efficiently vaporized under the same temperature flow, pressure and ambient (carrier) gas conditions, and (iv) the CVD deposition process can be performed using a fixed ratio of the CVD precursors in the solution, distinct advantage since the relative proportions of the respective components cannot be easily quickly changed.
In instances in which the aforementioned criteria (i)-(iv) are not fully met, and the precursors used as source reagents for the various film constituents are dissimilar in their liquid delivery (e.g., solution formation and vaporization) requirements, the use of the conventional liquid delivery system will be correspondingly sub-optimal in terms of the efficiency of the process and the structure and properties of the resulting deposited films.
It therefore is an object of the present invention to provide a liquid delivery. apparatus and process accommodating multiple precursors for deposition formation of a corresponding multicomponent material layer, where the precursors are dissimilar in their liquid delivery requirements.
Other objects and advantages of the invention will be more fully apparent from the ensuing disclosure and appended claims.